1. Field of the Invention
The present invention generally relates to a photodetector. More particularly, the present invention relates to a far infrared photodetector utilizing a mechanism of detection based on free carrier absorption and internal photoemission over the bandgap offset of a heterojunction. Still more particularly, the present invention is directed to heterojunction based terahertz detectors covering the range of from 1-60 THz. Further, the present invention is directed to a near- and far-infrared p-GaAs dual band detector, a near- and very-long-wavelength-infrared Si dual band detector, and GaN/GaN, GaN/AlGaN and InGaN/InGaN ultraviolet/infrared dual-band detectors that, in various embodiments, capable of detecting both UV and IR, simultaneously detecting UV and IR, and UV, IR or visible light.
2. Description of the Related Art
Far infrared (hereinafter “FIR”) detectors are of interest for various astronomy applications such as the Stratospheric Observatory For Infrared Astronomy (SOFIA) program and Explorer missions. Stressed Ge[1] (hereinafter “[n]” referring to the nth reference in the attached list of references at the end of the specification) and blocked impurity band[2] detectors have been studied for almost 20 years as FIR detectors without being successful in making large arrays. Due to the material constraints in Ge that limit their use in arrays Si and GaAs homojunction interfacial workfunction internal photoemission infrared photodetectors (hereinafter “HIWIP”) have been studied as an alternative detector structure.[3, 4] HIWIP detectors include successive highly doped emitter layers and undoped barrier layers. Detection takes place by free carrier absorption in the emitter layers followed by the internal photoemission of photoexcited carriers across the barrier and collection.[5] The threshold wavelength (λ0) is determined by the workfunction at the interface which is due to the bandgap narrowing caused by the doping in the emitter. By adjusting the device parameters, mainly the doping concentration in the emitter region, the threshold wavelength may be tailored to the desired range.
HIWIP detectors have shown high responsivity and good detectivity in this range. The workfunction in HIWIPs is due to the bandgap narrowing effect in the highly doped emitter regions. High density theory, where only the dopant type (n or p) is considered but not the specific impurities, has been used to calculate the workfunction associated with doping concentration.[5]
There is a need to develop a new type of far infrared photodetectors based on a new detection mechanism, giving high quantum efficiency. In a further aspect, terahertz imaging, in part due to its ability to differentiate between different non-metallic materials, is finding a wide range of applications from security applications to quality control in manufacturing. This makes the THz radiation imaging for screening for materials such as plastic explosives which would not show up on conventional x-ray screening. In another example, it also would allow inspection of computer chips for internal defects after processing. It is also contemplated that there are numerous potential medical applications ranging from, for example, cavity detection to scanning for skin cancers. THz radiation can even be used to enhance the recognition of watermarks for applications such as currency scanners.
Because many organic compounds have absorption features in the THz range, the identification of drugs and other compounds is another potential area of application. Another potential biological application is in using THz radiation to identify DNA structures. A further aspect of exemplary biological application is the extension of FTIR difference techniques to the 4-36 THz region to allow for the direct study of amino acids involved in metal binding in biological systems. This aspect impacts many studies of bio-molecular reactions, particularly in FTIR studies of pigment-protein systems that have been well characterized in the 36-60 THz spectral regions. For example, the visual protein rhodopsin, the proton pump bacteriorhodopsin, the respiratory cytochrome oxidases, ATPases, the iron containing proteins, hemoglobin and myoglobin, as well as a whole range of Cu, Ca, Ni, Fe, Mn, Mg, Zn, metal containing enzymes can be studied. Most of these metal containing complexes are involved in mammalian life processes, thus improved fast THz detectors could impact many areas of medical and biological research. Potential applications for biological monitoring include measuring water content in leaves, studies of tissue, identification of chemical compounds in samples, and the like. Currently known systems typically use pulsed systems. Thus, the development of continuous wave systems will potentially lead to cheaper alternatives for scanning applications.
All the presently available detectors in this spectral range have limitations on their operation. Conventional Schottky diode detectors have a high sensitivity on the order of 10−10 W/Hz1/2. However, typical intensity limits of 1 W restrict applications. Similarly, known Golay cells, which detect vibrations in a gas cell, are very sensitive to any type of mechanical vibrations and require additional vibration isolation. Thus, such Golay cell are not readily extended to array formats. In addition, being thermal detectors, they will have very slow response times. Further, although a Keating meter, which has a typical area of 30 cm2, will provide an absolute calibration, it is slow, requires a modulated signal (10-50 Hz), and cannot be converted into an array format with high resolution. While commercial thermopile detectors covering the THz range are available, they typically do not have D*>1010 Jones. And, while bolometer detectors can achieve high sensitivities (>105 V/W) by cooling to low temperatures, they are not very fast. Even microbolometers will have time constants on the order of milliseconds making them much slower than desired. Further, known Si blocked impurity band detectors operate at the high frequency end of this range (>7 THz) and stressed Ge:Ga detectors can operate as low as 1.5 THz. However, the stress requirements of the Ge:Ga detectors make them relatively unsuited for use in array formats. Recently, the range of Quantum Well Infrared Photodetectors (QWIPs) has also been extended to the high THz frequencies. Thus, although several THz detectors are available in the market, each has its limitations.
Thus, there is a need for a new photodetector.